1. Field of the Invention
This invention relates to a permanent magnet-type rotary electric machine and a production method for a rotor for the permanent magnet-type rotary electric machine. More particularly, the invention relates to a permanent magnet-type rotary electric machine capable of achieving high output by reducing eddy current, and a production method for a rotor for the permanent magnet-type rotary electric machine.
2. Description of the Related Art
A rotor of a permanent magnet-type rotary electric machine is formed, for example, as shown in FIG. 6. That is, a rotor core 50 is provided with a plurality of holes 52, and permanent magnets 54 are inserted into the holes 52, so that magnetic poles are formed. The rotor is rotationally driven on the basis of the rotating magnetic fields formed by electrifying the coils of a stator 60 disposed surrounding the rotor core 50. Since the permanent magnets 54 form magnetic fields separately for the individual predetermined magnetic poles, a small-size construction and high output can be achieved.
It is to be noted herein that since the magnetic fluxes produced by the coils of the stator 60 penetrate the permanent magnets 54, eddy currents occur in the permanent magnets 54. In the case where a permanent magnet 54 inserted into a hole 52 of the rotor core 50 is made up of a single body as shown in FIG. 6, there occurs, in the permanent magnet 54, an eddy current whose current path is wide and extends in the entire single-body magnet. The eddy currents produced in magnets give rise to conspicuous problems of heat generation and loss as the rotary electric machines are reduced in size, and improved in speed, and improved in output. Specifically, heat generation leads to the demagnetization of the magnets in a rotary electric machine, which becomes a cause of a failure of the rotary electric machine. Besides, the loss by the eddy current reduces the efficiency of the rotary electric machine.
Therefore, in the permanent magnet-type rotary electric machines, a reduction of the eddy current that occurs in a magnet is an important task, and structures of permanent magnets in which the eddy current is restrained are considered, for example, in Japanese Patent Application Publication No. 2000-324736 (JP-A-2000-324736), Japanese Patent Application Publication No. 2000-228838 (JP-A-2000-228838), and Japanese Patent Application Publication No. 2006-136130 (JP-A-2006-136130).
FIG. 7A is a diagram showing a structure of a permanent magnet in a permanent magnet-type rotary electric machine described in Japanese Patent Application Publication No. 2000-324736 (JP-A-2000-324736).
Referring to FIG. 7A, a permanent magnet 56 inserted into one of the holes 52 of the rotor core 50 is constructed of a plurality of permanent magnet pieces 560 arranged in a direction perpendicular to the rotation direction (which corresponds to the direction of the rotation axis). That is, the permanent magnet pieces 560 are equivalent to a plurality of permanent magnet pieces obtained by dividing the single-body permanent magnet 54 shown in FIG. 6. In the description below, the permanent magnet 56 in accordance with this structure will be simply referred to as “divided-type permanent magnet”.
An insulation material 562 for securing electrical insulation is interposed between adjacent permanent magnet pieces 560. The insulation material 562 used herein is, for example, an insulation paper, an insulation tape, or a synthetic resin such as epoxy resin or the like. Furthermore, each of the permanent magnet pieces 560 has been subjected to an insulation process in which each piece is coated with an insulation material (not shown).
FIG. 7B is a diagram showing eddy currents that occur in the permanent magnet 56 shown in FIG. 7A. Referring to FIG. 7B, the eddy currents flow in the individual permanent magnet pieces 560, along current paths shown by arrows. That is, the current path of eddy current changes from a single path that is the current path extending in the entire single-body magnet into a plurality of paths that correspond to the number of divisions of the permanent magnet 56.
According to this structure, the current path of eddy current is divided, so that the total sum of the divided lengths becomes greater than the length of the current path of the single-body magnet. Therefore, the electric resistance to eddy current becomes larger, that is, eddy current less readily flows, so that the eddy current loss and heat generation can be reduced.
It is to be noted herein that in order to effectively restrain the production of eddy current in the foregoing divided-type permanent magnet 56, there is a need to make small each one of the divided current paths of eddy current. To this end, there is a need to increase the number of divisions so that each one of the permanent magnet pieces 560 becomes sufficiently small.
However, when the number of divisions is increased, the volume occupied by the insulation material provided for each permanent magnet piece 560 enlarges, and the apparent intensity of magnetism declines. Therefore, there arises a problem of the output torque of the rotary electric machine declining in comparison with the single-body permanent magnet 54.
Furthermore, the divided-type permanent magnet also poses a problem in productivity, that is, the cost needed for the electric insulation process makes it difficult to reduce the production cost.
In detail, as for the divided-type permanent magnet 56 shown in FIG. 7A, the permanent magnet pieces 560 formed piece by piece need to be electrically insulated by the insulation materials 562 before being assembled into an integral-body permanent magnet. The cost associated with this insulation process has been a factor that inhibits the cost reduction. In particular, as for the foregoing effect of reducing the eddy current, the effect obtained becomes higher if the number of divisions of the permanent magnet 56 is greater and therefore the electric resistance to the eddy current is higher. However, increasing the number of divisions results in increasing the cost needed for the insulation treatment.
As a structure that eliminates the need for the insulation treatment, Japanese Patent Application Publication No. 2000-228838 (JP-A-2000-228838) discloses a permanent magnet that is divided into a plurality of permanent magnets so that adjacent permanent magnets are placed in partial contact due to slits (hereinafter, the permanent magnet in accordance with this structure will be simply referred to as “slit-type permanent magnet”).
However, the slit-type permanent magnet, too, has a problem as follows. That is, since there is a need to increase the total number of slits in order to effectively restrain the occurrence of eddy current, the proportion of the slits to the entire volume of the permanent magnets becomes large, so that the output torque of the rotary electric machine declines.
Furthermore, since the processing for slits requires considerations for avoiding impairment of the strength of the permanent magnet, the production steps are complicated, and it becomes difficult to expect a great effect in reducing the cost.
Furthermore, Japanese Patent Application Publication No. 2006-136130 (JP-A-2006-136130) discloses a permanent magnet that is constructed so that electrical insulation between permanent magnet pieces is secured by gap portions that are formed at end portions of adjacent permanent magnet pieces when a plurality of permanent magnet pieces are stacked.
According to this permanent magnet, in the case where the magnetic flux density at the foregoing end portions is relatively high and therefore eddy current flows concentratedly in the end portions, the electric resistance in the end portions becomes high and therefore the eddy current can be effectively reduced. However, in the case where the magnetic flux density is substantially uniform, there occurs a problem of being unable to reduce the eddy current that flows in the portions other than the foregoing end portions.